1. Field of the Invention
The present invention relates to a differential amplifier circuit.
2. Description of the Related Art
As differential amplifier circuits which accurately amplify and have a wide dynamic range, a folded-cascode differential amplifier circuit is generally used. FIG. 6 is a diagram showing a configuration example of a general folded-cascode differential amplifier circuit. A folded-cascode differential amplifier circuit 100 is configured to include a pair of differential transistors 101, and cascode transistors 102 connected to the differential transistors 101 in a folded cascode manner.
The differential transistors 101 are configured by N-channel MOSFETs (M1, M2), in which sources of the N-channel MOSFETs (M1, M2) are connected to a constant current source 103, a drain of the N-channel MOSFET (M1) is connected to a constant current source 104, and a drain of the N-channel MOSFET (M2) is connected to a constant current source 105. The differential transistors 101 perform a differential operation corresponding to an input voltage VIN+ applied to a gate of the N-channel MOSFET (M1) and an input voltage VIN− applied to a gate of the N-channel MOSFET (M2).
The cascode transistors 102 are configured by P-channel MOSFETs (M3, M4), in which a source of the P-channel MOSFET (M3) is connected to the drain of the N-channel MOSFET (M1), and a source of the P-channel MOSFET (M4) is connected to the drain of the N-channel MOSFET (M2). Gates of the P-channel MOSFETs (M3, M4) are applied with a bias voltage VBIAS lower than a power supply voltage VDD by a certain voltage, and a current corresponding to the differential operation of the differential transistors 101 is outputted from drains of the P-channel MOSFETs (M3, M4).
Herein, IE denotes a value of a current of the constant current source 103; I denotes those of the constant current sources 104 and 105; IM1 denotes that which passes in the N-channel MOSFET (M1); IM2 denotes that which passes in the N-channel MOSFET (M2); IM3 denotes that which passes in the P-channel MOSFET (M3), and IM4 denotes that which passes in the P-channel MOSFET (M4). When the input voltages VIN+ and VIN− are equal, a relationship of IM1=IM2=IE/2 is established. Therefore, a relationship of IM3=IM4=I−IE/2 is established. When there is a difference between the input voltages VIN+ and VIN−, relationships of IM1=IE/2+i and IM2=IE/2−i are established, where i denotes an amount of current change of the differential transistors 101 corresponding to the difference. Therefore, relationships of IM3=I−IE/2−i and IM4=I−IE/2+i are established. Thus, from the drains (output electrodes) of the cascode transistors 102, currents obtained by amplifying the difference between the input voltages VIN+ and VIN− are outputted.
Incidentally, in the differential amplifier circuit 100, a source voltage VS of the N-channel MOSFETs (M1, M2) changes corresponding to a voltage level of the input voltages VIN+ and VIN− applied to the gates of the N-channel MOSFETs (M1, M2), that is, a voltage level of a common mode voltage VCM in the input voltages VIN+ and VIN− On the other hand, drain voltages VD of the N-channel MOSFETs (M1, M2) are a constant voltage determined by the bias voltage VBIAS applied to the gates of the P-channel MOSFETs (M3, M4). That is, drain-to-source voltages VDS of the N-channel MOSFETs (M1, M2) changes corresponding to the voltage level of the common mode voltage VCM.
To increase frequency response of the differential amplifier circuit 100, it is effective to shorten channel lengths of the N-channel MOSFETs (M1, M2). However, when the channel lengths of the N-channel MOSFETs (M1, M2) are shortened, it becomes susceptible to influence of a channel length modulation. FIG. 7 is a graph showing one example of a relationship between the drain-to-source voltage VDS and a drain current ID. As shown in FIG. 7, due to the influence of the channel length modulation, the drain current ID gradually increases along with an increase of the drain-to-source voltage VDS even in a saturated region. Thus, when the drain-to-source voltage VDS changes, the drain current ID also changes. When the drain-to-source voltage VDS further increases, a punch through phenomenon in which a current passes irrespective of a gate voltage occurs.
Therefore, in the differential amplifier circuit 100, when the drain-to-source voltages VDS of the N-channel MOSFETs (M1, M2) change corresponding to the voltage level of the common mode voltage VCM, the values of currents IM1 and IM2 which pass through the N-channel MOSFETs (M1, M2) change, and those of currents IM3 and IM4 outputted from the P-channel MOSFETs (M3, M4) also change. That is, output currents IM3 and IM4 change corresponding to the voltage level of the input voltage (common mode voltage), and as a result, accuracy of the differential amplification decreases.
The present invention has been achieved in view of the above-described problem, and an object thereof is to provide a differential amplifier circuit capable of stably performing a differential amplification irrespective of an input voltage level.